Fix path resolution of rustfmt in cargo-fmt

[onur-ozkan]

Instead of resolving rustfmt path from RUSTFMT or PATH envs, use the one located in the same directory as cargo-fmt. So that cargo-fmt uses the expected version of rustfmt to avoid accidental use of unrelated versions.

Fixes #137666

[rustbot]

r? @Mark-Simulacrum

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[rustbot]

Some changes occurred in src/tools/rustfmt

cc @rust-lang/rustfmt

[Kobzol]

Now that I think about it, I did some local testing again, and I think that the current behavior sort of... makes sense? cargo-fmt is from the nightly toolchain, but it seems like its intended behavior indeed should be to resolve rustfmt from either RUSTFMT or PATH, even though that rustfmt might actually be from a different toolchain.

And when it is invoked through cargo +nightly fmt, everything works as expected (it invokes the nightly rustfmt). So I think that my original issue was actually a red herring and it's fine. But I'll let Mark take a look.

[onur-ozkan]

FWIW clippy also follows this approach.